﻿The <em>IfcGridAxis</em> needs to be used by exactly one of the three attributes of <em>IfcGrid</em>:
<ul>
  <li><em>UAxes</em></li>
  <li><em>VAxes</em></li>
  <li><em>WAxes</em></li>
</ul>
i.e. it can only refer to a single instance of <em>IfcGrid</em> in one of the three list of axes.